This invention relates to apparatus for burning gaseous fuel.
The modern domestic gas burner has evolved from the discoveries of three men.
Robert Wilhelm Bunsen, a German chemist, invented the first high temperature blue flame burner in 1855 which is still in use in laboratories. The basic principle of a jet of gas entraining air in a tube open at both ends has its counterparts in all gas burners with the exception of the jet and target burners.
Sir Humphrey Davy, an English chemist, discovered that a gas flame would not pass through a wire screen providing the spacing between the wires was less than a specific size. This discovery led to his invention of the miner's safety lamp before 1819. This phenomenon is the basis of all burners which have burner heads with many small ports. Some laboratory burners still use a wire mesh for preventing the flame to flash back to the gas nozzle orifice. Flashback will always occur with burner ports which are larger than the Davy critical size whenever the efflux velocity of the air-gas mixture has a magnitude that is less than the burner velocity of the gas. This phenomenon can also occur if the temperature of the ports reaches a sufficiently high value that they raise the air-gas mixture temperature. The burning velocity of the air-gas mixture increases with the temperature and the result can be a flashback. The flashback tendencies become minimal when the depth of the ports is three-eighths inch or more. Likewise, means for cooling the ports reduces the flashback tendencies. The ports, however, which are of small diameter introduce a resistance to the flow of the air-gas mixture. This resistance if the flow is laminar increases inversely with the fourth power of the port diameter. The effect of this port head loss is to reduce the capacity of the burner. All of the energy for entraining and mixing the air with the gas must be intrinsic to the jet of gas at the nozzle orifice exit. Although the method of mixing the gas in a straight tube as originated by Bunsen is still used today in some commercial gas burners, there is a superior method for mixing air and gas as follows.
Giovanni Batista Venturi was an Italian physicist who invented, before 1822, a device which is named after him, that is, a Venturi tube. This well-known device is used for measuring fluid flow, as an ejector, etc.,; is in effect a nozzle upstream from the throat and a diffuser downstream from the throat. A superior burner is thus obtained by replacing the Bunsen tube with the Venturi throat and diffuser. A portion of the velocity head of the air-gas mixture entering the burner throat is transformed into static pressure in accordance with the Bernouli equation. This static pressure thus compensates for the resistance to flow of the gas ports with the result that the burner capacity is increased accordingly.
The highest performance obtained by a gas burner embodying the above components is the laboratory Meker burner in which all of the air for complete combustion is mixed with the gaseous fuel before the air-gas mixture leaves the burner port. Such a burner is variously called a stoichiometric burner, a fully aerated burner or a burner with 100% primary air. In addition to the highest flame temperature, the gas products can impinge on a cold surface without producing carbon monoxide because the combustion is complete at the tips of cones at the flame base. However, such a burner has a very limited output capacity. There are no fully aerated domestic gas burners in existence and the reasons for their nonexistence will be explained later.
All domestic gas burners use primary and secondary air, including jet and target gas burners. The Bunsen-Venturi gas burners use primary air ratios of about 30% to 60%. The range top burners have the higher primary aerations and the ovens use the lower aerations with heat outputs of 9000-12000 BTU/hr for the former and 18000-24000 BTU/hr for the latter. Water heaters use about 30000 BTU/hr and some of the burners are of the jet and target type. My calculated aerations for the latter type indicate that the primary aerations are less than 60% as determined by the jet orifice diameter and the distance between the jet and the target.
These partially aerated gas burners have many disadvantages including much lower flame temperature, the inability of the flames to impinge on cold surfaces without carbon monoxide formation, large combustion space for secondary aeration, and so forth.